Overfilling theory (input needed)
 

Mind you, this is only a theory that I would like the general opinion from Fuelly users, who tend to be more mpg driven and conscious than the general population.

As the adult son of a 40+ years mechanic, who also tinkers in his own cars, I completely understand why overfilling the gas tank is a bad thing. But it strikes me that it would only be bad on an everyday scenario, and specially if you overfill the tank and then park your vehicle for several hot summer days (please don't do that).

My theory is: If you are about to take a looooong road trip, lets call it El Paso to San Antonio, If you overfill your fuel tank as the last thing you do before leaving town, wouldn't the "extra" fuel you are carrying be used-up before it has a chance to evaporate, and since you are driving at highway speed, the catalitic converter would be able to burn off any additional fumes as it is intended to anyway? Also on said road trip, wouldn't you be able to get some additional range?

If you do it now and then, I'm sure there's no harm. I did it every fill for over a year and I'm lucky that it didn't damage my canister. The only reason I did it was to provide extra accuracy on my fillups. I decided the risk wasn't worth the extra accuracy or the extra ~.8 gallons of extra fuel.

I always brim mine until you can physically see fuel sitting right by the cap itself. It says in the manual to stop after the third click, but I'm so obsessed with getting the most miles out of my tank, Im not fussed about any risks. I like being able to do 320 miles before the fuel gauge starts to drop.

You might find people that do this are the same people that drive forever once the fuel low warning light appears too, yep I'm guilty of that aswell, I generally leave it a week or so when the light pops on, some people believe that running fuel that low is bad for your car too. Like you say, there are lots of theories out there, some prove people right, some prove people wrong it all depends if you want to risk it or not. I personally don't feel there's any harm in practicing either of these methods.

most vehicles have a"charcoal cannister" that catches the vapor (fuel) & can be "saturated" do not know if that is thecase of your vehicle, I usually stop when the cutoff stops the flow of fuel - good practice?

I'd say it's good practice to stop at the first click. It's why your filler cap and the gas pump will say do not top off. As I said, I miss the extra range and the (probably, maybe) more accurate fill data from stuffing the tank, but it's not worth the possibility of having to prematurely replace the canister. In mine, it's a $300 part for a car worth maybe $800.

Not sure if diesels even have a canister? Diesel doesn't evaporate like gasoline does.

It really doesn't matter very much whether you top off the tank or not, as long as you are reasonably consistent. Fill the tank till the nozzle shuts off and call it good; fill it to the next convenient unit of currency (dollar, in the USA); or waste time trying to get the very last drop in the tank. It will all average out within a few tanks anyway. Your day-to-day variations in driving, the weather changes over even a few days - or worse, over a month - will have far more effect. Seasonal variations in fuel mix, coupled with the uncertainty of what you get (is the service station tank all winter blend, all summer blend, or some unknown mixture? how much ethanol is in it?) make a variation of a fraction of a gallon unimportant.

Just hate to fill at a gas station. Fill the tank up!!! That's why diesel VW's were so tempting. 900 highway miles before you need filling. Was interested in an ECO Cruze, which had a false specification of a 12.6 gallon tank, but really had a 15.6 gallon tank. Carefully driven highway range would be 700+miles. Tho my car was bad-mouthed for bad mpg, I've been able to average 39mpg & keep the range above 400 miles, which is nice. Fill the tank up!!!

Elantras are fantastic hypermilers. Doesn't take much more effort to keep its range over 500mi. Great cars!

I see the discussion seems to have shifted from getting the most accurate mpg figure for a given fill to getting the longest range on a fill. For range, the semi I used to drive averaged about 6 mpg but had a range exceeding 1100 miles at mostly highway speeds. I never ran it dry and usually filled it about once per day at somewhere around 500 miles. Its secret to long range was simple - two saddle tanks holding over 100 gallons each.

The efficiency of large trucks has significantly improved since then, it's not unusual to get 12 to 17 MPG in a large semi these days, and the long life oils used can do 100,000 km between changes, better for the environment all round. There's also less need for such big tanks, around here, fuel is so dear, trucks had to have armour plated tanks to stop people drilling them and siphoning it off. A tank that size fully loaded with fuel could be worth up to $2000 her, easy target.

I'd like to see your source for that mileage. We have European engines in some of our trucks, too, and they do NOT get that sort of mileage. If they did, trucking companies would switch to them in a heartbeat.

4-6mpg is all you'll see in a semi. Maybe closer to 8ish unloaded

Sorry, I keep forgetting the difference, I was quoting UK MPG, and that is what the truck drivers tell me they get. At the lower end you have the big Semi trucks, whilst the 18 tonne trucks get slightly better.

Hi,

From a Hypermiling perspective, filling the tank to the brim means that you get further on each tank full, over time this means that you have to stop less times overall for the same distance. Stopping and starting have a big impact on MPG, hence the struggle to squeeze every last mile out of each fill.

For example on our Skoda, from the first click to the brim adds about 2-3 litres, which is probable good for 50km or more, we get around 600km to a tank, so just undeer 10% extra, so it effectively saves 1 fill in 10-12 fills.

Is it worth it? Probably not, one extra stop/start every 6000km.

It's dangerous to fill and leave filled, definitely for Petrol which expands etc in the heat. I normally fill up on the way home from work and have another 20km to drive after filling, so I've already used a litre or so of the extra fuel.

As for getting accurate MPG figures, makes no difference long term. If however you are driving in an MPG competition or using a borrowed car for a review, it is absolutely crucial that the tank is brimmed at the start of the test and then brimmed at the end to get an accurate figure for the fuel used.

Oliver.

I agree Oliver, keeping a constant speed is very important on long journeys. Having to stop and fill up every few hundred miles, and then having to accelerate back up to cruising speed is bound to use more fuel than staying at a steady speed for the entire journey. Having said that, carrying the extra weight of a full tank could use slightly more fuel than carrying a quarter tank, especially in a hilly region.

I could be wrong, but I think most folks are going to have to stop every two or three hours to empty their bladders. The basic reason for long range fuel capacity would involve travelling somewhere without convenient fuel stops, such as sparsely populated regions. However, if long range is important to you, have at it.

Diesel's have no evap system. My 2011 Golf TDI I always filled to the brim. My 2015 XV Crosstrek Subaru gasser, first click and I'm done. Not going to take any chance on killing the charcoal canister.

I'd like to amend this statement...I have seen as high as 8.5 in my truck, fully loaded, but that's if I keep it rolling for 4+ hours straight on decent roads. 7.5-8mpg is more common. Urban driving kills these trucks' fuel economy, much more than any car I have driven on a % basis.

Unloaded, you'll get about 11mpg. Bobtailing I have seen 12-14.

If you're shifting, you're wasting fuel in a semi.

Numbers above have been from a '14 Freightliner Cascadia, DD13/500hp engine with tag axle and Eaton 10-speed over the last 40k miles. Lightweight model, has a slightly shorter sleeper and only one fuel tank. The company limits speed to 60mph on cruise, 63mph hammer down.

I put the nozzle in the filler as far as it will go. I fill on the medium setting of the 3 notches. When it shuts off automatically I manually fill until the next automatic shutoff, so one overfill. Then I shut it off and go on my way. I believe the extra 1/3-1/2 gallon will be absorbed by air gaps and other normal things and will not damage my car. I would not go beyond one extra cutoff though.

I think the manual to a past car said to fill to the second click. Possibly to account for the first click being triggered by frothing.

I use the a lower fill setting when available. Some stations might only have have one.

For monitoring fuel economy, pick a fill method and stick with it for better precision in the calculations.

I've never seen froth/foam from a gas tank fill. Now diesel, holy foaming Batman.

Ah...so you carried an extra one hundred gallons(700 pounds of diesel) that you never used. If you would have carried 100 fewer gallons, your mpg would have been higher.

One of the things I missed about owning a diesel was the ability to vent and top off to the tippy top with no worries allowing for a 14 gallon tank in my VW TDI to hold almost 15 gallons. This also allowed for the most precise measurements of mpg since one can see the fuel right there at the cap and one sees it each time at the same spot, knowing the possible filling error has been eliminated.

However, diesel fuel, like most liquids can and will expand when heated up, i.e like parking out in the sun as the earth warms up during the day, so the only worry regarding a diesel and topping up, as long as one tops up carefully while watching the fuel level at the pump so as not to inadvertently spill it on the pavement, would be if you vented and topped up and then didn't drive far enough on a cool morning before parking to burn off a little bit of fuel. In that scenario, some fuel could spill out around the cap or overflow system in the vehicle, but that would be a rare scenario for most drivers.

I always brim the tank as you explained, best way to get accurate results, although we are relying on the pump calibration for that, even though we know we get the same amount in every time as you can see the fuel in the filler, the reading on the pump might not be that accurate.

I fill at a slow rate and after the first click I'm done - may not be the most accurate but beats spilling raw fuel onto the ground (expansion, take a turn too fast etc...)

In my xB if I stop at the first click I still have a gallon of space in the 11.9 gallon tank and since I don't have a very big tank and don't want to test the 1.9 gallons reserve when the low gas light comes on I try to fill it pretty full. That last gallon can take about 5 more minutes of pump a little and wait for it to flow into the tank but it's like 10% more fuel in the tank since I have to start thinking about getting more gas when the light comes on at pretty much exactly 10 gallons used since full full. I do notice quite a variation on how accurate the pump gallons match the scanguage gallons used but once in a while I find a gas station pump that is within a ten of a gallon fillup after fillup. One thing that bugs me is how the scanguage continues to "use" gas after the ignition is turned off especially if I rev the engine just before I turn off the ignition. Why? Because flushing the engine with fresh air actually makes it start a lot easier next time ... Like on the second compression stroke.

You can apply the exact same argument to everyone else who tops off the tank instead of carrying only the fuel needed for the trip.

By the way, airlines make a fine art of carrying only the fuel needed for their trip, plus legally mandated reserves.

It's also dangerous for planes to land with more weight than necessary, in an emergency situation when the plane has not long been in flight, they have to dump the fuel before landing or the plane will break up if it's fully laden with fuel.

The 100 gallons of "extra" fuel weight is nothing compared to the maybe 40,000 lbs of payload he could be hauling. Let's keep things in perspective!!